ATPCO, the airline-owned company that distributes fare data, puts it simply: a fare is the charge to fly from origin to destination, and the rules are the conditions of travel for that fare. It sorts them into 32 numbered categories, including:

• Category 5, advance reservations and ticketing: for example how far in advance the ticket must be bought.

• Categories 6 and 7, minimum and maximum stay: for example a Saturday night stay or a return within 30 days.

• Category 10, combinations: how the fare can be combined with others, including for open jaws.

• Categories 16, 31 and 33, penalties, voluntary changes and voluntary refunds: the charges and conditions for changing or refunding a ticket.

The fare basis code identifies the fare, but its full conditions are in the rules.

Why they matter for travel businesses

• Agents must follow them. Virgin Atlantic, for example, requires bookings on its tickets to comply with all applicable fare rules and bans back-to-back tickets used to get round rules such as advance purchase or minimum stay. When a ticket breaks the rules, its general approach is to charge the difference to the next fare that meets all the conditions, through an agency debit memo.

• Fares on sale must include them. UK law says fares offered to the public for flights from UK airports must include the applicable conditions, and the final price must always be shown.

• They let airlines price differently. IATA says rules such as using flight coupons completely and in sequence are what allow airlines to offer different fares to different market segments.

Worth knowing

IATA notes that passengers on the same flight will have paid a range of fares under different conditions, so the rules of one fare do not tell you the rules of another.